I think it would be the tail gunner also. If the Luftwaffe pilots knew a B-17's tailgun position was out, no other gunner position could fire on them, he could stay above the ball turret's line of fire, but behind that tall tail , so the upper gunner couldn't fire, and neither waist gun could fire close to straight back.
As long as he stayed in that cone that could only be fired on by the tailgunner, he could stay back there and fire at his leisure, no deflection problems for him to figure out either.
The only fire he'd have to worry about is the other B-17s.